The present invention is in the field of optical waveguides and principally relates to optical waveguide structures useful as mode scramblers, mode couplers, or the like.
The modal propagation of light in optical waveguides has been discussed by Hicks et al. in U.S. Pat. No. 3,157,726, and by N. S. Kapany in "Fiber Optics--Principles and Applications", Academic Press (1967). As discussed in these and other publications, the propagation of light waves in optical waveguides is governed by laws of physics similar to those that cover microwave propagation in waveguides, and therefore can be studied in terms of modes, each of which has its own propagation and electromagnetic field characteristics.
Single mode waveguides are advantageous in that they are capable of propagating optical signals with very low dispersion, but due to the low numerical aperture and/or small core size of such fibers, it is difficult to efficiently couple optical signals into these waveguides. Multimode waveguides have larger core diameters and/or larger numerical apertures than single mode waveguides. Multimode waveguides are therefore often the preferred medium for the transmission of optical signals since they can accept light from incoherent, broad spectral width sources such as light emitting diodes.
Thousands of modes propagate in multimode optical waveguides, each mode travelling at a slightly different group velocity. A short input pulse that propagates via many guided modes thus splits up into a sequence of pulses that arrive at the output end of the waveguide at different times. This type of pulse dispersion, termed modal dispersion, is the dominant cause of dispersion in typical multimode optical waveguides.
The earliest optical waveguides consisted of a core of uniform refractive index surrounded by a layer of cladding material having a lower refractive index. In this type of waveguide, termed a step-index waveguide, the time required for the various modes to travel a given longitudinal distance along the waveguide increases as the mode order increases. The delay distortion in such a fiber, defined as the difference in the times taken by the fastest and slowest modes to traverse a given waveguide length, is very large, so that the usable bandwidth of the light signal is reduced.
It has recently been recognized that optical waveguides, the cores of which have radially graded index profiles, exhibit significantly reduced pulse dispersion resulting from modal velocity differences. This dispersion-reducing effect, which is discussed in the publication "Multimode Theory of Graded-Core Fibers", D. Gloge et al., Bell System Technical Journal, Pages 1563-1578, November 1973, employs a radially graded, continuous index profile from a maximum value on-axis to a lower value at the core-cladding interface. The index distribution in this type of waveguide is given by the equation: EQU n(r)=n.sub.1 [1-2.DELTA.(r/a).sup..alpha. ].sup.1/2 for r.ltoreq.a (1)
where n.sub.1 is the on-axis refractive index, a is the core radius, .DELTA.=(n.sub.1.sup.2 -n.sub.2.sup.2)/2n.sub.1.sup.2, and n.sub.2 is the refractive index of the fiber core at radius a. An optical waveguide of this type, called a graded-index waveguide, exhibits very low modal pulse dispersion when the value of .alpha. in the above equation is near 2, and consequently exhibits a much higher usable bandwidth than a step-index waveguide.
A graded-index optical waveguide having an .alpha. value on the order of 2, such that the core index depends strongly on core radial position, is an example of a type of waveguide hereinafter referred to as a low-alpha waveguide. An optical waveguide resembling a step-index waveguide, having an invariant core index or a core index varying only slightly with core radius, is an example of a type of waveguide hereinafter referred to as a high-alpha waveguide. In terms of the above equation, a true step-index optical waveguide is taken to be one wherein alpha has an infinite value.
A major difficulty in characterizing the information-carrying capacity or bandwidth of optical fiber waveguides stems from a lack of standardization in the measurement procedure. A fundamental problem arises in relation to the distribution of waveguide modes initially excited by the testing light source. This so-called "launch condition" depends on the angular and spatial distribution of light from a selected source which is initially incident upon the end of the waveguide. In a typical light source such as a semiconductor laser diode, the light issuing from the source and injected into the waveguide core is not uniformly distributed as to injection position or injection angle. The result is that only certain waveguide modes are initially excited by the pulse of light. As a consequence of this nonuniformity, pulse dispersion and the resulting bandwidth value reported for the waveguide will vary strongly depending upon the particular light source selected for testing and the core location at which the light pulse is injected into the waveguide.
A proposed solution to the problem of bandwidth measurement reproducibility is to utilize a mode scrambler, also called a mode mixer, which mixes or mode-couples light passing into or through a waveguide so that a more uniform spatial and angular distribution of light proceeds down the waveguide core. Among the mode scramblers utilized in the prior art are those which rely on microbending-induced mode coupling effects, such as the mode scrambler described by M. Ikeda et al. in Applied Optics, Vol. 16, No. 4, Pages 1045-1049 (1977), or the mode scrambler described by M. Eve et al. at the Second European Conference on Optical Communications, Tour Olivier de Serres, Paris, 1976, Part 2, Communication V.3.
A mode scrambler utilizing sinusoidal fiber bending is discussed by M. Tokuda et al. in Electronics Letters, Vol. 13, No. 5, pp. 146-147 (1977), and a mode exciter incorporating an etched fiber end is described by M. Ikeda et al. in Applied Optics, Vol. 15, No. 9, Pages 2116-2120 (1976). J. P. Hazan et al. suggest the use of a step-index fiber as a "distributed ray scrambler" in the Philips Technical Review, Vol. 36, No. 7 (1976) at page 213.
In general, mode scramblers utilizing microbending coupling or etched end diffusion effects present problems of device reproducibility, whereas most other scrambler configurations are cumbersome and inconvenient to use.